A New Deepkill Eurypterid

Abstract

While engaged in a restudy of the stratigraphic distribution of graptolites in the type section of the Deepkill shale, Rensselaer County, N. Y., the writer discovered the small eurypterid described below. The form is of interest in that it is the first occurence in the Canadian strata of a eurypterid having the aspect of the Silurian genus Eusarcus. The generic position of this species is dubious. The prosoma is typical of Eusarcus, which is typically developed in the Silurian. Other members of the genus have been described from rocks of Trenton and Cincinnatian age. However, the present species occurs in beds which are certainly not younger than the Canadian, and are probably Middle Canadian, if not older, for graptolites homotaxial to those of the overlying graptolite beds 3-5 of the Deepkill Section occur in the Powell limestone of the Ozark region. With few exceptions, the body and appendages of Ordovician eurypterids are poorly known. A very large pterygotid, soon to be described by Dr. K. E. Caster from the Richmond of Manchester, Ohio, has a prosoma typical of Pterygotus, yet the remainder of the animal is so different in body pattern that a new genus must be erected for it. This indicates further the need of caution, in generic determination of Ordovician Eurypterida on the basis of prosomal characters alone. Three eurypterids have been previously described from the Deepkill. One of these, Pterygotus deepkillensis Ruedemann (1934) is from approximately the same horizon as the present species; that is, from the zone characterized by horizontal Didymograptus species, and represented in the Deepkill Section by beds 1 and 2. The other two, Dolichopterus antiquus Ruedemann and Pterygotus (?) priscus Ruedemann (1942) are from the beds exposed at the Ash Hill quarry at Mt. Merino, south of Hudson, N. Y. Ruedemann (1904, p. 499), on the basis of their fauna regarded the fossiliferous beds at this exposure as intermediate in position between beds 5 and 6 of the Deepkill Section. Some of the eurypterids found in graptolite shale occurred under such conditions as to suggest that they might have been only cast skins washed into the black shale facies. (Reudemann 1934, p. 40-41). The occurrence of eurypterids in a shale layer directly on top of Normanskill grits, with some specimens on the surface of the grit layer itself, suggested deposition of the materials during transition from strong to weaker currents. The specimen described in this paper was found under very different conditions. The type is from a layer of soft and very fine-grained black shale three-fourths of an inch in thickness. The material was evidently fine-grained mud and contains remarkably large and perfect specimens of Tetragraptus fruiticosus, Goniograptus thureaui var. postremus, and several species of Didymograptus. The underlying shales assume a slightly coarser texture for